Typically, submerged gravity type separators have been used for cleaning gravel and the like. Typical of such devices is my U.S. Pat. No. 3,411,627. In such devices, the material to be cleaned is lighter than the material from which it is to be separated. Thus, such devices are concerned with recovering the bottom layer which is produced in the stratification process.
In recent years, with the resurgence in popularity of coal as a fuel, it has also been necessary to provide the coal in a reasonably clean state due to various environmental and economic factors. It is therefore an object of this invention to provide a device which is capable of removing the undesirable materials from the raw coal and which is capable of handling the large volume of material from the top layer to stratification. Of the two layers which result from the stratification, the previous devices have been concerned with cleaning and removing the bottom layer. The instant invention on the other hand cleans the top layer.